THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP01/10993.
The present invention relates to a high-frequency amplifier for use in a communications device such as a mobile telephone.
A conventional high-frequency amplifier shown in FIG. 14 generally includes a transmission line 1403 having one end connected between an amplifier circuit 1401 and an output matching circuit 1402 and the other end connected to a power source 1405 and is grounded via a capacitor 1404.
The transmission line 1403 is designed to have xc2xc the wavelength of a frequency band of a signal be amplified by the amplifier circuit 1401. The capacitance of the capacitor 1404 is set to a level great enough to short-circuit in the frequency band.
A bias current received from the power source 1405 is a direct current and thus does not flow through the capacitor 1404 but flows through the transmission line 1403 to drive the amplifier circuit 1401. The amplifier circuit 1401 amplifies a signal in the frequency band and may simultaneously generate harmonic distortions in a band of n times greater than the frequency band (where n is an integer). The capacitor 1404 is short-circuited in the frequency band and the length of the transmission line 1403 is equal to xc2xc the wavelength. As a result, the transmission line 1403 is inverted in phase at one end and stays open, thus allowing the amplified signal to be received not by a bias circuit 1406 but by the output matching circuit 1402.
In the frequency band of 2n times greater than the band, the capacitor 1404 is short-circuited. Also, the length of the transmission line 1403 is equal to xc2xd the wavelength. Therefore, the bias circuit 1406 functions as a notch circuit. As a result, the harmonic distortions at 2n times the frequency band can be attenuated and hardly received by the output matching circuit 1402.
FIG. 15 illustrates a frequency response of a conventional high-frequency amplifier including an amplifier circuit 1401 having an output impedance of 3.2-j5.7xcexa9 in an output frequency band of 900MHz.
Since the bias circuit of the conventional high-frequency amplifier develops an insufficient short-circuit across the capacitor at the frequencies of second and third harmonics, a low pass filter needs to be connected after the matching circuit for attenuating the harmonic distortions. This increases the overall size of the circuit, and reduces the operating efficiency of the conventional high-frequency amplifier due to a loss of the added low pass filter; thus increasing current consumption for producing a desired level of power.
A high-frequency amplifier includes (a) an amplifier circuit, (b) an output matching circuit, and (c) a bias circuit. The bias circuit includes a parallel circuit constructed by a first capacitor and a first transmission line having a first end connected between the amplifier circuit and the output matching circuit and a second end connected to a power source, and a second capacitor having a first end connected to a second end of the parallel circuit and a second end grounded.
While remaining open in a frequency band of a signal to be amplified by the amplifier, the bias circuit is short-circuited in a desired frequency band. This allows undesired harmonic distortions in the signal to be attenuated without using a low pass filter.